A central challenge in cancer immunology is to explain inter-individual differences in spontaneous and immunotherapy-induced immunity, and then build on the explanatory mechanisms to enhance existing and develop novel immunotherapies. Based on studies across diverse cancers, a substantial increase in overall survival is observed for patients with higher densities of T cells in their tumors. However, what remains obscure is why some patients develop powerful immune responses while others have undetectable immunity. A series of recent studies reported that patients with high loads of tumor mutations are more likely to have durable responses to checkpoint blockade therapy ? for MSI+ CRC, lung cancer in smokers, and melanoma. The current hypothesis is that more mutations generate more neoantigens that provide unique targets for T cells to recognize tumors. Since mounting a strong immune response also requires stimulation of specialized pathogen sensors that drive innate immune response, we have hypothesized that potent tumor immunity may also depend on engagement of pathogen sensors. We recently discovered that damaged DNA is exported from nucleus to cytosol where it triggers the STING DNA-sensing pathway and thus induces cytokines, chemokines and subsequent immune responses, a finding observed recently by several groups independently. We propose that tumors with higher loads of damaged DNA could trigger intrinsic innate immune responses via STING and enhance protective anti-tumor immunity. Further supporting this hypothesis, we have identified 50 cancer cell lines that express a STING-dependent innate immune response constitutively. We thus hypothesize that tumors with higher loads of damaged DNA (or mutation rates) trigger DNA sensors within tumor cells, and induce innate immune responses that drive T or NK cell rejection of the tumor. This hypothesis synergizes well with the hypothesis that higher mutation rates produce more neoantigens, and explains the induction of both innate and adaptive immunity as a function of mutation rates and DNA damage. We propose to comprehensively test the role of damaged DNA in driving tumor immunity, using a combination of cell culture studies to study the role of damaged DNA in driving innate immune response (Aim 1); a mouse model to determine the impact of damaged DNA within a tumor on STING-dependent immune rejection of the tumor (Aim 2); and studies of human colorectal cancers and melanomas to test for associations between damaged DNA, local tumor immunity and clinical outcome (Aim 3). Since our long-term goal is to discover the mechanisms that explain variations in tumor immunity, we will employ an unbiased approach (Aim 3.2) to generate new hypotheses for how tumors drive or suppress immunity in human tumors with known outcome. Our studies are expected to help explain inter-individual variation in tumor immunity, address why immunotherapy succeeds or fails to control tumors, and inspire novel therapeutic targets.